linux-kernel - Re: [PATCH V2 1/8] mm/highmem: Lift memcpy_[to|from]

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Message-ID: <BYAPR04MB49655E6A3BD935B15DCC468B868D9@BYAPR04MB4965.namprd04.prod.outlook.com>
Date:   Wed, 10 Feb 2021 06:33:24 +0000
From:   Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>
To:     "ira.weiny@...el.com" <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Sterba <dsterba@...e.cz>
CC:     Boris Pismenny <borisp@...lanox.com>,
        Or Gerlitz <gerlitz.or@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        "hch@...radead.org" <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Eric Biggers <ebiggers@...nel.org>, "clm@...com" <clm@...com>,
        "josef@...icpanda.com" <josef@...icpanda.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V2 1/8] mm/highmem: Lift memcpy_[to|from]_page to core

On 2/9/21 22:25, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
>
> Working through a conversion to a call kmap_local_page() instead of
> kmap() revealed many places where the pattern kmap/memcpy/kunmap
> occurred.
>
> Eric Biggers, Matthew Wilcox, Christoph Hellwig, Dan Williams, and Al
> Viro all suggested putting this code into helper functions.  Al Viro
> further pointed out that these functions already existed in the iov_iter
> code.[1]
>
> Various locations for the lifted functions were considered.
>
> Headers like mm.h or string.h seem ok but don't really portray the
> functionality well.  pagemap.h made some sense but is for page cache
> functionality.[2]
>
> Another alternative would be to create a new header for the promoted
> memcpy functions, but it masks the fact that these are designed to copy
> to/from pages using the kernel direct mappings and complicates matters
> with a new header.
>
> Placing these functions in 'highmem.h' is suboptimal especially with the
> changes being proposed in the functionality of kmap.  From a caller
> perspective including/using 'highmem.h' implies that the functions
> defined in that header are only required when highmem is in use which is
> increasingly not the case with modern processors.  However, highmem.h is
> where all the current functions like this reside (zero_user(),
> clear_highpage(), clear_user_highpage(), copy_user_highpage(), and
> copy_highpage()).  So it makes the most sense even though it is
> distasteful for some.[3]
>
> Lift memcpy_to_page() and memcpy_from_page() to pagemap.h.
>
> [1] https://lore.kernel.org/lkml/20201013200149.GI3576660@ZenIV.linux.org.uk/
>     https://lore.kernel.org/lkml/20201013112544.GA5249@infradead.org/
>
> [2] https://lore.kernel.org/lkml/20201208122316.GH7338@casper.infradead.org/
>
> [3] https://lore.kernel.org/lkml/20201013200149.GI3576660@ZenIV.linux.org.uk/#t
>     https://lore.kernel.org/lkml/20201208163814.GN1563847@iweiny-DESK2.sc.intel.com/
>
> Cc: Boris Pismenny <borisp@...lanox.com>
> Cc: Or Gerlitz <gerlitz.or@...il.com>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Suggested-by: Matthew Wilcox <willy@...radead.org>
> Suggested-by: Christoph Hellwig <hch@...radead.org>
> Suggested-by: Dan Williams <dan.j.williams@...el.com>
> Suggested-by: Al Viro <viro@...iv.linux.org.uk>
> Suggested-by: Eric Biggers <ebiggers@...nel.org>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>

Thanks for adding a new line in the new calls after variable declaration.
Looks good.

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@....com>